feat(installer): add WriteInstallManifestStep

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Installer/Steps/WriteInstallManifestStep.cs

@@ -0,0 +1,25 @@
+using System.IO;
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Steps;
+
+public sealed class WriteInstallManifestStep : IInstallStep
+{
+    public string Name => "Write Install Manifest";
+
+    public Task<StepResult> ExecuteAsync(InstallContext ctx, IProgress<string> progress, CancellationToken ct)
+    {
+        if (string.IsNullOrWhiteSpace(ctx.InstalledVersion))
+            return Task.FromResult(StepResult.Fail("Installed version is not set — DownloadAndExtractStep must run first."));
+
+        var manifest = new InstallManifest(
+            Version: ctx.InstalledVersion!,
+            InstallDir: ctx.InstallDirectory,
+            WorkerDir: Path.Combine(ctx.InstallDirectory, "worker"),
+            InstalledAt: DateTimeOffset.UtcNow);
+
+        InstallManifestStore.Write(ctx.InstallDirectory, manifest);
+        progress.Report($"Wrote {InstallManifestStore.ManifestPath(ctx.InstallDirectory)}");
+        return Task.FromResult(StepResult.Ok());
+    }
+}